1. Field of the Invention
This invention relates to an automotive stall circuit to de-energize an ignition coil in an automotive ignition system whenever the automotive engine rpm is below a predetermined operating frequency. More particularly, the stall circuit provided de-energization of the coil at the stall frequency without causing causing spark to the engine yet allowing operation of the engine and the ignition system to produce spark below the stall frequency.
2. Description of the Prior Art
It is necessary, in a high-energy adaptive dwell electronic ignition system comprising an integrated circuit, an ignition coil and discrete power circuitry as is generally known to those skilled in the art, to shut down the ignition system whenever the engine speed decreases below a predetermined stall frequency in order to protect the coil and discrete power circuitry. During normal engine operation the adaptive dwell ignition system controls the amount of power that is dissipated in the coil and power circuitry. However, at engine speeds below the stall frequency, e.g., one hertz, the time between successive engine sparking is great enough to allow excessive power dissipation in the coil and discrete power device which could very well damage or destroy either the coil or the power circuitry or both. Hence, almost all contemporary high energy adaptive dwell electronic systems have some type of stall circuit associated therewith to cause shutdown of the ignition system at the stall frequency to prevent damage of the coil and the power circuitry. For example, U.S. Pat. No. 4,100,907 as well as commonly assigned U.S. patent application, Ser. No. 253,423 entitled "START TO RUN CIRCUIT FOR AN ELECTRONIC IGNITION SYSTEM", describe an adaptive dwell ignition system having stall circuits for shutting down of the ignition system at engine rpm's below a predetermined speed.
A problem with some prior art stall circuits is that once the stall circuit de-energizes the coil, the ignition system is rendered inoperative until the engine is restarted by turning the key in the ignition switch. Thus, these prior art ignition systems do not continue to operate below the stall frequency to continue to produce engine spark therebelow. This prevents push starting of a stalled automobile including the type of ignition systems as aforedescribed.
Therefore, a need exists for a stall circuit for softly de-energizing the ignition coil of an automobile at the stall frequency to prevent a spark from being generated yet allowing the engine and ignition system to operate below this frequency to provide correct engine spark with a minimum amount of spark retard.